Methane-fueled internal combustion engines are attractive alternatives to liquid-fueled spark-ignition and compression-ignition (diesel) engines for many applications in which reduced exhaust emissions are required. In addition, economy of operation favors the use of methane-fueled engines in many specific applications. Methane-fueled engines are subject to similar exhaust emission regulations as conventional liquid-fueled engines in both stationary and mobile applications. Such emission requirements include control of carbon monoxide, unburned hydrocarbons, and certain oxides of nitrogen, generally defined as NO.sub.x, which in the art typically refers to NO (nitric oxide) and NO.sub.2 (nitrogen dioxide). In mobile applications, i.e. cars, trucks, and buses, the most common emission control method for gasoline engines is the well-known three-way catalyst which typically yields a 90% reduction in NO.sub.x, CO, and unburned hydrocarbons.
The use of three-way catalysts in natural gas-fueled automotive engines is described in European Pat. Application Publication No. 0 468 556 A1. In applications of natural gas-fired stationary engines for power generation, pipeline pumping, compression, and the like, a three-way catalyst can be used as described in several references including "Catalytic Control of NO.sub.x Emissions from Stationary Rich-Burning Natural Gas Engines" by K. R. Burns et al, ASME Paper 83-DGP-12; "Reduced NO.sub.x Emissions from Internal Combustion Engines Fuelled by Natural Gas" by T. Fowler et al in Fuel, 1991, Vol. 70, April, pp. 499-502; "Control of NO.sub.x /CO/HC Emissions from Natural Gas Fueled Stationary Engines with Three-Way Catalysts" by J. C. Summers et al, presented at the 84 th Annual Meeting and Exhibition of the Air and Waste Management Association, Vancouver, B.C., Jun. 16-21, 1991; and U.S. Pat. No. 5,131,224. The removal of NO.sub.x from a rich-burn gas-fired engine by nonselective catalytic reduction by CO and unburned hydrocarbons is described by C. Castaldini et al in an article entitled "Environmental Assessment of Catalytic Reduction of Natural-Gas-Fired Engines" in Proc. Sympos. Stationary Combustion NO.sub.x Control 1985, Vol. 2, pp 57/1-57/16, 1987. The article also discusses the use of selective catalytic reduction by ammonia for NO.sub.x control in a lean-burn gas-fired engine.
Methane also is used in combination with other components in fuels for internal combustion engines, and such engines are also defined herein as methane-fueled engines. B. Nagalingam et al describe the use of hydrogen-natural gas mixtures as well as pure hydrogen for stationary engine fuel in an article entitled "Performance Study Using Natural Gas, Hydrogen-Supplemented Natural Gas, and Hydrogen in AVL Research Engine" in J. Hydrogen Energy, Vol. 8, No. 9, pp. 715-720, 1983. The use of hydrogen or hydrogen-natural gas mixtures were found to decrease unburned hydrocarbons but increase NO.sub.x in the exhaust gas. R. W. Hum also describes the use of hydrogen-methane mixtures as fuel for spark-ignition engines in a paper of the same title in Proc. Conf. Natur. Gas Res. Technol., 2nd, 1972, VI 4, pp. 1-8. Low BTU gas comprising methane, hydrogen, and carbon monoxide can be used for engine fuel as reported by H. M. Kosstrin et al in Energy Biomass Wastes, 1988, Vol. 11, pp. 217-236 in an article entitled "The Combustion of MSW-Derived Low-BTU Gas in an Internal Combustion Engine". Such gas can be produced by fluid-bed gasification of municipal solid waste (MSW) to yield a product typically containing 5 vol % methane, 14 vol % carbon monoxide, and 6 vol % hydrogen. This gas after appropriate cleanup was used successfully to operate a gas-fired engine driving an electric generator. Exhaust emissions were controlled by a dual-bed catalytic system in which NO.sub.x was removed in a reducing environment, and CO and unburned hydrocarbons were removed under oxidizing conditions using air injection. Mixtures containing methane and CO.sub.2 as fuel for a four-stroke spark-ignition engine were tested and reported by J. K. S. Wong in a paper entitled "Study of Mixtures of Methane and Carbon Dioxide as Fuels in a Single-Cylinder Engine" in Mech. Eng. Report-Natl. Res. Council Canada, MP, MP-70, 27pp., 1976 . Such mixtures are obtained from gaseous products of anaerobic biological decomposition such as landfill gas, sewage digester offgas, and the like.
Methane can be used as fuel for a diesel engine wherein a small amount of ordinary liquid diesel fuel is injected into the cylinders with the methane to ensure proper ignition. The operation of such an engine, generally defined as a dual-fuel engine, is described in a paper entitled "Operating a Diesel Locomotive With Liquid Methane Fuel" by J. L. Stolz, ASME Energy Source Technology Conference and Exposition, Houston, Tex., Jan. 26-30, 1992. Development work is in progress as well on the operation of diesel engines fueled with methane alone. The use of methane to replace most or all of the liquid diesel fuel reduces particulate emissions, but control of NO.sub.x is still required. Since combustion occurs on the lean side in a diesel engine, the only current practical catalytic method for NO.sub.x control in conventional and dual-fuel diesel engines is selective catalytic reduction with ammonia. While exhaust gas recirculation provides a degree of NO.sub.x reduction, this noncatalytic method cannot meet the low emission requirements anticipated in the future.
Currently practiced catalytic NO.sub.x removal methods for emission control in methane-fueled internal combustion engines have several drawbacks. First, the use of the three-way catalyst is effective only when the engine is operated at carefully-controlled air/fuel ratios, generally slightly fuel-rich and in a very narrow operating window, so that the three-way catalyst is not applicable to highly efficient lean-burn engines which utilize compression-ignition (diesel) or spark-ignition cycles. Such lean-burn engines, which exhibit significantly lower fuel consumption, generally cannot achieve low NO.sub.x emissions by catalytic methods except for selective catalytic reduction by ammonia. Second, three-way catalysts cannot control both CO and NO.sub.x at very low levels simultaneously, since the CO and NO.sub.x react over the catalyst resulting in a tradeoff in control of these two combustion products. Third, in order to achieve very low NO.sub.x levels by reduction with ammonia, a large excess of ammonia is required which increases the discharge of ammonia in the treated exhaust. In addition, ammonia is a hazardous material which requires careful handling, and is impractical in mobile applications.
Emission regulations for both mobile and stationary internal combustion engines will become more stringent in coming years. Methane-fueled engines will be increasingly attractive for their favorable emission characteristics as well as economy of operation, and improved methods for more stringent control of NO.sub.x emissions from these engines will be required. Lean-burn engines will be favored in applications where fuel efficiency is important, but simultaneous control of carbon monoxide and NO.sub.x in a single catalyst system is not possible given the current state of the art in lean-burn engine emission control. The present invention disclosed in the following specification and defined in the appended claims offers a novel and useful method for the control of NO.sub.x and carbon monoxide emissions from methane-fueled engines, especially lean-burn engines, by means of a single catalyst system.